YMMV / My Little Pony: Friendship Is Magic S3 E13 "Magical Mystery Cure"
Accidental Innuendo: Spike's "Gangnam Style" hopping, which Psy explained as a "horse-riding dance", seems to suggest that Spike likes "riding" ponies...and the mares in Sugar Cube Corner thought it was cute.
Anvilicious: The show is never exactly subtle about the joys of friendship, but here, they hammer it in hard. Though some fans still consider the episode better for it.
Base-Breaking Character: Alicorn Princess Twilight Sparkle. The idea was controversial before the episode even aired. When it finally aired, the general consensus was split between it being a good thing, a terrible thing, or that it was too early to tell. After season 4 made it clear that she still had the same Adorkable personality, detractors calmed down a bit. But the alicorn ascension still stands as the most drama-inducing moment in the show's history even to this day.
Broken Base: Hoo, boy. Opinion is divided over how well the episode pulled off the above; whether it felt rushed and filled with too many songs instead of explaining events, or if this was a good way of condensing the plot down into a single episode without sacrificing the story-telling.
Fanfic Fuel: Becoming Alicorn Princess is now something that can be earned?!
How exactly did the spell change the Mane Six minus Twilight's backstories to result in them having each other's cutie marks? One particularly popular theory is that Pinkie Pie married Big Macintosh.
Harsher in Hindsight: Celestia's statement that Star Swirl the Bearded "did not understand friendship like [Twilight] does". The comic series reveals that Celestia and Star Swirl were friends and their friendship fell apart when he caught her traveling through a magical mirror without his permission.
Holly Giesbrecht, an animator on the show and the creator of background (and later toyline) pony Holly Dash, drew this bit of Side-Story Bonus Art early on in the show's life, the sketch was posted about a month after the first episode aired. What was originally some cute fanart from one of the animators (with the colored version mysteriously taken off of her DeviantArt page) ended up being some epic Foreshadowing.
The same applies to the countless number of Alicorn Twilight fan fics and pictures that existed before this episode was announced.
Getting back to the original conflict, you would not believe the number of "Twilight-screws-up-a-spell" fanfictions that were made in the past two years. It was considered cliche before the actual show got a chance to use it.
Pinkie Pie receiving Applejack's cutie mark takes on a slightly new light after "Pinkie Apple Pie" toyed with the idea that they could be distantly related and even though nothing was confirmed by the end of that episode, it didn't deny it either.
When news of the Brony phenomenon began to reach the major news networks during the series' first season, Wait Wait... Don't Tell Me! brought on former US President Bill Clinton to play "Not My Job" and answer questions about the show. note As a former POTUS, Clinton is arguably one of the most influential and relevant people in the world, while a show for little girls like MLP must be, they reasoned, one of the least relevant topics. This episode literally has five of the Mane Six playing "Not My Job".
It's Short, So It Sucks: The main complaint on the episode (after the initial kerfuffle about Alicorn Twilight) seems to be that it felt a little rushed and should have been a two-parter. Most importantly, it was never explained what Twilight's change would mean. A tweet from Meghan McCarthy states that there will be two more episodes which will turn this episode into a three-parter. Thosetwo episodes did address this by setting up a Half-Arc Season which ultimately answered those critiques. M.A. Larson also said he wanted it to be a two-parter, but Executive Meddling told him no.
It Was His Sled: Months after the episode aired, the severe fandom reaction to Twilight becoming a princess (with individual reactions ranging from excitement to outrage to mere shock) had become so widespread and memetic that any newcomer to the fandom could immediately know about Twilight's ascension without even watching the episode.
Never Live It Down: Poor M. A. Larson. He has written some of the best episodes in the series, but his name is forever tied to the ensuing Broken Base regarding Twilight becoming an alicorn. Several seasons have passed and fans still yell "LARSON" (Though often mockingly) whenever alicorns are mentioned or a major change in the Status Quo is in the works.
Worse still: Twilight fixed her mistake. Star Swirl couldn't. Did he test this spell, and, if so, how many lives did he ruin by mistake?
Scapegoat Creator: M. A. Larson spoke out about the episode, and emphasized his original script for it was significantly different, most importantly he wanted it to be a two-part episode, and he wrote it with the understanding this was the series finale. Then the script was taken out of his hands, shrunken down into one episode, and some of the removed plot elements were moved into Season 4's premiere. Yet, some fans blame him and him alone for all this episode did.
Spoiled by the Format: You know there's going to be more coming when it looks like the problem's solved and everything's going to be all fine and dandy with a third of the episode left.
Squick: The way Pinkie's legs bend when she tries to applebuck a tree during the Cutie Mark song... ow.
Suspiciously Similar Song: "Find a Way" sounds like a slowed-down rendition of "Major Tom (I'm Coming Home)," particularly the delivery of the lines "Losing promise, I don't know what do/Seeking answers, I feel I won't get through."
Tainted by the Preview: An "audio only" version of one of the songs in the episode, A True True Friend, was leaked on Internet five days before the episode aired, with it not having mixed reactions from people at Equestria Daily, but also hinted that the "swapped Cutie Marks" plot would be rushed for the sake of having Twilight become an alicorn in the final minutes.
They Changed It, Now It Sucks: The criticism to Twilight becoming a Alicorn Princess boils down to this since it's a seemingly major, possibly permanent physical change to the main character.
The Mane Character's roles get swapped? Wait, what do you mean that it doesn't happen to the main Mane Character? We barely get to see results before Twilight figures out how to solve the problem, since the third act is needed for Alicorn Twilight. As noted above, some fans think this should have been a two-parter.
Some fans were a bit dismayed that Twilight flew into the camera at the very end because an episode about her learning how to use to her new wings would have been nice. While her future appearances show her struggling to fly well, it's only as a minor at most plot point.
The show has thus far only given hints as to how exactly Cutie Marks work and how much they signify. This would have been the perfect opportunity to go into this subject but once again they don't, except by implication. Plus, there's also the fact that the dilemma was solved with the most obvious and direct solution possible instead of having to make them work for the solution.
Visual Effects of Awesome: Special attention has been paid to the lighting of this episode: there are several beautiful instances of shading, and some lens flares have been added. Add to this some dynamic camera movements (particularly the first shot, imitating the opening theme song), the epic shot of Rarity struggling to control a storm during The Cutie Mark Song, some particularly smooth animation, especially on Alicorn Twilight, and the gorgeous Celestial Plane, and "Magical Mystery Cure" is the best-looking episode up to this point of the series by a long shot.